The present invention relates to processes for preparing olefin catalysts and, more particularly, to a method of synthesizing a novel class of alpha-olefin polymerization catalysts comprising titanacarbocycles.
The combination of metal alkyls of Groups 1, 2 or 13 metals of the Periodic Table with metal halides, or salts of Groups 4 or 5 metals at ambient temperatures in hydrocarbon media, has long been known to function as an effective catalyst system for the polymerization of olefins, conjugated dienes, or acetylenes. These catalysts were discovered and reported by Karl Ziegler and coworkers in 1952. The resulting catalysts formed in a heterogeneous phase are further capable of the stereoregular polymerization of alpha-olefins, 1, 3-alkadienes and alkynes, as was observed and reported by Giulio Natta and coworkers in 1953. The revolutionary impact that these dual discoveries exerted on the polymer industry worldwide has been evident in the thousands of patents issued in the last 50 years that describe so-called Ziegler-Natta technology and in the joint conferral of the Nobel Prize in Chemistry on these two pioneers in 1963. An admirably thorough review of the scientific and patent literature of Ziegler-Natta polymerization technology before 1978 has been published (J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, New York, 1979).
The specific interaction of either lithium alkyls (RLi) or aluminum alkyls (R3A1) at room temperature with titanium(IV) chloride can lead to the exchange of one or two chloro ligands of TiCl4 (1) for alkyl groups of RLi (Eqs. 1 and 2) to yield alkyltitanium derivatives 2 and 3: 
The polymerizing action of the heterogeneous reaction mixture of TiCl4 and RLi has been ascribed either to compounds 2 and 3, which precipitate from the reaction mixture along with the by-product LiCl, and/or to the TiCl3 (4) or TiCl2 (5) generated by the homolyses of 2 and 3 (Eqs. 3 and 4). 
Many studies and patents corroborate that preformed TiCl3, combined with a main-group metal alkyl activator, and preformed TiCl2, with or without a metal alkyl activator, can effect the polymerization of ethylene and of alpha-olefins, as well as the isotactic polymerization of alpha-olefins. All such polymerizations apparently occur in a heterogeneous phase and are proposed to ensue by repeated insertions of the olefin monomer (7) into a preexisting carbon-titanium bond (e.g., 6), where the oxidation state of the titanium, n, could be 2, 3 or 4 (Eq. 5). 
In light of the novel catalyst system described herein, it is important to note that a hydrocarbon solution of titanium(IV) chloride (in alkane or arene medium), when treated at room temperature with 2 equivalents of n-butyllithium (cf. Eq. 2) immediately takes on a brown color and over a period of an hour rapidly darkens as a black solid precipitates. Finally, the supernatant liquid is colorless, and solid TiCl2 and LiCl have formed quantitatively, as analyses have shown (Eq. 6). Thus any intermediate, such as di-n-butyltitanium 
dichloride 8a has completely decomposed to 5 under these conditions. With reference to the works of Friedlander and Oita, who polymerized ethylene (Ind. Eng. Chem., 49, 1885 (1957)), and of Max Frankel and coworkers, who polymerized ethylene and propylene (J. Polymer Sci., 28, 387 (1958) and 40, 149 (1959)) by forming their catalyst by combining various ratios of TiCl4 and BuLi at ambient temperatures, it is most likely that what was actually being generated was either TiCl2 or TiCl3 as the active catalysts, and not 8a. Moreover, these previous workers generally prepared their catalysts and conducted their polymerizations at room temperature and under an atmosphere of nitrogen gas. Our laboratory has now shown that catalysts such as 8a are not stable at 25xc2x0 C. under nitrogen but are destroyed by the nitrogen, ultimately reducing the nitrogen to ammonia. Of necessity, therefore we prepare catalyst 8a and 8b at xe2x88x9278xc2x0 C. and under an argon atmosphere.
The present invention describes the synthesis of a novel family of olefin polymerization catalysts that can be readily generated by the preparation and utilization of di-n-butyltitanium dichloride (8a) or di-t-butyltitanium dichloride (8b), that are generated as a suspension with LiCl at xe2x88x9278xc2x0 C. in alkane, cycloalkane, or aromatic hydrocarbon media (Eq. 7). 
Under these conditions, 8 is sufficiently stable to undergo a remarkable, unprecedented reaction whereby the TiCl2-moiety of 8 is efficiently transferred to a substrate like diphenylacetylene (9), to generate three-membered titanacycle 10 (Eq. 8), most likely via the octahedral transition state 11. 
Similar reactions will be discussed hereinafter with reference to intermediates comprising such titanacyclopropanes or titanacyclopropenes. These olefin catalysts comprise a class of. three-membered titanacarbocycles, which have high activity and stereoregularity in olefin polymerization, and can be easily prepared from di-n-butyltitanium dichloride or di-t-butyltitanium dichloride when combined with either olefins or acetylenes through a novel process termed epimetalation-by-transfer and exemplified in Eq. 8.
Some advantages of synthesizing and using these novel catalysts are the following: they are relatively inexpensive to synthesize because they can be generated from commercially readily available starting materials; their synthesis is straightforward because the catalyst preparation uses common laboratory or pilot-plant apparatus; the structure of the specific catalyst can be varied widely by changing the nature of the specific acetylene, olefin or diolefin employed; polymerization proceeds with acceptable reaction rates, even at lower temperatures and pressures; polymerization of alpha-olefins may be made to proceed in an isotactic manner; and the inventive catalyst system can be employed for the copolymerization of two or more olefin monomers.
The only possible disadvantage of the present inventive catalysts is their sensitivity to traces of moisture and active-hydrogen organic compounds like alcohols, acids, 1-alkynes and to oxygen, or peroxides, all of which can destroy the carbon-titanium bond and thereby inactivate the catalyst. However, such chemical sensitivities are shared with other Ziegler-Natta polymerization catalysts as well.
Titanacycles are the key polymerization intermediates formed when a dibutyltitanium dichloride (8a or 8b) is generated by mixing titanium tetrachloride and butyllithium together (Eq. 7), then exposing them to either ethylene or propylene gas. Such titanacycles, in hexane or toluene solution, but in the absence of THF, initiate the polymerization of ethylene into polyethylene and of propylene into isotactic polypropylene.
To the best of present knowledge and belief, polymerization catalysts for olefins and other related compounds having titanacycles as an essential part of their structure have not been previously reported.
In accordance with the present invention, a novel family of olefin polymerization catalysts comprising three-membered titanacarbocycles is illustrated. The titanacarbocycles can be readily synthesized by utilizing di-n-butyltitanium dichloride or di-t-butyltitanium dichloride, prepared at xe2x88x9278xc2x0 C. in alkane, cycloalkane, or aromatic hydrocarbon media. These procatalysts exhibit sufficient stability below 0xc2x0 C. that they can undergo a remarkable, unprecedented reaction whereby the titanium dichloride moiety is efficiently transferred to a substrate, such as ethylene, an alpha-olefin, acetylene or a mono- or disubstituted acetylene, or similar compounds, in order to generate the corresponding three-membered titanacarbocycles.
It is an object of this invention to provide a new class of olefin polymerizing catalysts.
It is another object of the present invention to provide novel polymerization catalysts for olefins that can be conveniently and inexpensively synthesized.
The present invention features a novel family of olefin polymerization catalysts comprising three-membered titanacarbocycles. The titanacarbocycles can be readily synthesized by utilizing di-n-butyltitanium dichloride or di-t-butyltitanium dichloride prepared at xe2x88x9278xc2x0 C. in alkane, cycloalkane, or aromatic hydrocarbon media. These catalysts exhibit sufficient stability after their preparation that they can undergo a remarkable, unprecedented reaction whereby the titanium dichloride moiety is efficiently transferred to a substrate like ethylene, an alpha-olefin, acetylene or a mono- or disubstituted acetylene, or similar compounds, in order to generate the three-membered titanacarbocycles of the type previously shown in Eq. 8.
The novel catalyst system in all its variants can be prepared from readily available, commercially produced starting materials by use of apparatus present in any well-equipped research laboratory or industrial pilot-plant.
The catalyst can be generated and used in the heterogeneous phase for low-pressure, low-temperature polymerization of individual olefins, copolymerization of two or more olefins and diolefins, and for the isotactic polymerization of alpha-olefins without the use of expensive and less accessible metallocene or structurally complex nonmetallocene procatalysts.
Presently, it is believed that this epimetalation-by-transfer occurs via transition state 11 (Eq. 8), in which reductive elimination of the butyl groups is induced by coordination of substance 9. Proof the structure of 10 was achieved by treatment with D2O and the isolation of 12 (Eq. 9). 
Now pertinent to this invention is the finding that suspensions of 10 in alkanes are highly active polymerization catalysts for ethylene and other olefins even at xe2x88x9278xc2x0 C.
Similar to the behavior of diphenylacetylene (9), depicted in Eq. 8, the corresponding titanacycles like 10 can be efficiently generated from acetylenes, such as di-n-butylacetylene, bis-trimethylsilylacetylene, and methyl(phenyl)-acetylene. In addition, olefins such as the cis-isomers of 1, 2-disubstituted alkenes may also be converted to their respective titanacycles. All such titanacycles exhibit olefin polymerization activity. The simple olefins, such as ethylene, propylene, 1-hexene and styrene possess unusual potential for this novel catalyst system. Suspensions of 8 in hexane at xe2x88x9278xc2x0 C. caused the immediate polymerization of either ethylene or propylene with the propylene being polymerized principally to isotactic polypropylene.
Owing to the unusual reactions undergone by 8 with acetylenes such as 9 and other olefins, it is reasonable to conclude the remarkable polymerization activity and stereoregulation exhibited by solutions of 8 and simple olefins is not simply due to some ill-defined interaction of the olefin monomer with the heterogeneous catalyst surface. Rather it would be logical to conclude that an epimetalation-by-transfer reaction as depicted in Eq. 8 and as illustrated for ethylene in Eq. 10 must play the key role in these polymerizations. Owing to the highly polar, strained carbon-titanium bond present in the proposed titanacycle 13, ethylene insertion into 13 and ensuing polymerization should be rapid. 
The generation of such a titanacycle in the case of propylene (14, R=Me) and other alpha-olefins (Eq. 11) would provide an excellent model for understanding the isotactic polymerization observed with these titanacyclic catalysts. For steric reasons, insertion of a second alpha-olefin (14a, shown in Eq. 12) would have the monomer approach bond a from the underside and with the R-group projecting away from the ring. Such an insertion would lead to 15 (Eq. 12). Again, for steric reasons, succeeding alpha-olefins would approach the growing ring (e.g., 15) at bond a and from the underside, leading to an isotactic ordering of the R-groups. 
In order to corroborate the necessary presence and intermediacy of these three-membered titanocycles, 13 and 14, they were respectively generated by treating 8a dissolved in tetrahydrofuran (THF) with ethylene gas (Eq. 13) or with propylene gas (Eq. 14), starting at xe2x88x9278xc2x0 C. and then warming up to 25xc2x0 C. Under these the titanacyclopropanes 13 and 14 were formed but did not initiate the polymerization of the individual olefins. It is well known that various titanium(IV) Ziegler olefin polymerization catalysts are completely inactivated by stoichiometric amounts of strong Lewis bases, such as ethers or amines. It can be reasonably assumed, therefore, that in THF 13 and 14 are intermediates kinetically stabilized as bis(tetrahydrofuran) complexes, 13xe2x80xa22THF and 14xe2x80xa22THF. Because of the 1-butene formed from Bu2nTi Cl2 (8a) in the epimetalation of ethylene, considerable amounts of the 1-butene-epimetalated product (16) are also produced (Eq. 13). 
When di-t-butyltitanium dichloride (17) was employed to epimetalate ethylene in THF, intermediate 13 was formed much more efficiently and in fact underwent further insertion of ethylene to produce 18. Intermediate 18 was trapped with benzonitrile to yield valerophenone (19) upon hydrolysis (Scheme 1). 
A proof of the presence of 13 in THF solution has been achieved by chemical trapping of 13 by successive reactions with benzonitrile (20) and then with carbon dioxide (22). Hydrolytic work-up and the isolation of benzoylpropanoic acid (24) is conclusive evidence that reaction intermediates 21 and 23 were involved and that the structure of the starting titanium compound must be 13 (Scheme 2). In a similar but abbreviated chemical trapping, the presence of 14 in THF solution prepared as in Eq. 14 was demonstrated by chemical trapping with benzonitrile (20), followed by hydrolysis (Scheme 3). 
The isolation of only n-propyl phenyl ketone (26) clearly indicates that benzonitrile inserted selectively into the sterically more accessible C-Ti bond a forms 25. Insertion into the more sterically hindered C-Ti bond (b) would have resulted in the formation of isopropyl phenyl ketone (27), a product not observed.
In summary, a novel class of olefin polymerization catalysts has been disclosed, viz, three-membered titanacarbocycles exhibiting high activity and stereoregularity in hydrocarbon media, which are readily prepared from Bu2nTiCl2 or Bu2tTiCl2 and either olefins or acetylenes through epimetalation-by-transfer.